Duncan Coral

The Duncan coral is recognized by its daisy-like polyps that open at the tips of long, tube-shaped corallites. The center of the polyp is gray-blue and the short tentacles around it are green; the tentacles stay open day and night, and when the colony is fully open they cover the skeleton. The corallites bud from the sides over time, so a small single-headed frag turns into a lush cluster within a few years.

It is a rare species in the wild; all of its popularity in the hobby comes from how easily it is propagated in the aquarium. As a stony coral, its trade is subject to CITES Appendix II; the vast majority of Duncans sold are aquacultured frags anyway, so choose those. Its eagerness to catch food makes it ideal for beginners who want to get into the habit of feeding corals.

• Green Duncan: The standard form with a gray-blue mouth and green tentacles.
• Ultra or purple Duncan: Rarer forms with purple, pink or reddish tones in the mouth and tentacles.

It lives in northern Australia and the South China Sea, attached to hard substrate in sand-dominated areas usually deeper than 20 meters. Living in deep water explains why it prefers moderate rather than strong light in the aquarium.

• Light: Low to moderate, about 50–150 PAR. In too much light the polyps shrink and do not open fully.
• Flow: Low to moderate and indirect. The polyps should sway gently; in strong flow they retract into the corallites.
• Placement: The middle or lower zone of the rockwork; a rock just above the sand is also suitable.
• Water parameters: Salinity 1.024–1.026, KH 7–10 dKH, calcium 400–450 mg/L, magnesium 1250–1350 mg/L. It is a tolerant species, but rapid changes in KH make the polyps close. A little nitrate (2–10 mg/L) and phosphate help keep the tissue full.

It is photosynthetic, but it is one of the most eager feeders among LPS. Two or three times a week, place mysis, brine shrimp, chopped shrimp or LPS food on each polyp with a pipette; the polyp closes over the food within a few seconds. Regularly fed colonies produce new heads noticeably more often. Feed with the flow turned down.

It is considered peaceful: it does not put out long "sweeper" tentacles, and its tentacles are short with a weak sting. Even so, it can lightly sting a neighbor it touches directly or be harmed by a more aggressive coral; leave about 5 cm of space around it. Do not place it within reach of aggressive species such as Euphyllia, Galaxea and elegance coral.

Clownfish sometimes host in a Duncan. Dwarf angelfish and some hermit crabs may pick at the polyps; otherwise it lives without problems alongside most reef fish.

It is very easy to propagate. One or several corallites are cut from the base of the cluster with coral cutters and glued to a frag plug. Side heads formed by budding can be taken individually; each corallite can survive on its own. The mouth of a cut corallite may stay closed for a few days and then opens normally.

• Polyps not opening: The most common cause is strong flow or a recent move; if it does not improve within a few days, check the light and KH.
• Tissue recession at the base of the corallite: Too much light, starvation or detritus collecting at the base of the corallite can cause it.
• Shrinking polyps: In a colony that goes unfed for a long time the polyps shrink; regular feeding corrects this.